https://help.gosimpletax.com/hc/en-gb/articles/360000151164-Voluntary-Class-2-NIC
You can elect to make voluntary class 2 voluntary contributions
Click on VALIDATE MY TAX RETURN
Click on YES
Answer YES to the question DO YOU WANT TO MAKE CLASS 2 VOLUNTARY CONTRIBUTIONS
Tick the box to pay the maximum amount (£153.40- 2018/19)
or Enter the voluntary contribution you need to make ( max £153.40)
Click on REVIEW CALC & PROCEED to save changes